The present invention relates to electric motors, and in particular, to brushless DC spindle motors used for rotating disks within disk drive data storage devices.
The latter half of the twentieth century has been witness to a phenomenon known as the information revolution. While the information revolution is a historical development broader in scope than any one event or machine, no single device has come to represent the information revolution more than the digital electronic computer. The development of computer systems has surely been a revolution. Each year, computer systems grow faster, store more data, and provide more applications to their users.
The extensive data storage needs of modem computer systems require large capacity mass data storage devices. While various data storage technologies are available, the rotating magnetic rigid disk drive has become by far the most ubiquitous. Such a disk drive data storage device is an extremely complex piece of machinery, containing precision mechanical parts, ultra-smooth disk surfaces, high-density magnetically encoded data, and sophisticated electronics for encoding/decoding data, and controlling drive operation. Each disk drive is therefore a miniature world unto itself, containing multiple systems and subsystem, each one of which is needed for proper drive operation. Despite this complexity, rotating magnetic disk drives have a proven record of capacity, performance and cost which make them the storage device of choice for a large variety of applications.
A disk drive typically contains one or more disks attached to a common hub or spindle, which are rotated at a constant speed by an electric motor, often called a spindle motor. The electric spindle motor is typically a brushless DC type motor, meaning that it contains a multiple phase stator and a permanent magnet rotor, the different phases of the stator being energized in a sequence by control electronics to impart a torque to the rotor. The sequential switching and driving of the different stator phases can induce mechanical vibrations at harmonic frequencies of the motor driving frequency of the disks. These vibrations may be audibly perceived by a user as a high-pitched whine.
A whining noise emitted by a disk drive storage device can be very annoying or distracting to a user, interfering with user productivity. Additionally, constant excitation at a particular frequency may have other undesirable side effects, such as possible interference with servoing and possible degradation of mechanical components.
Disk drive designers have long recognized the desirability of eliminating any spurious noise or vibration in the drives. Great attention is paid, therefore, to precision machining of mechanical parts and other methods and processes for controlling noise. However, noise abatement is not perfect, and there remains a need to find improved methods of reducing noise in disk drives, and in particular, noise at harmonic frequencies of the disk motor driving frequency.
In accordance with the present invention, an electric motor, which is preferably a spindle motor of a disk drive, contains a multi-phase stator driven by switching the phases of drive current at corresponding commutation angles of rotation of the rotor. Small variations, herein called semi-random offsets, are introduced into the commutation angles at which the drive current is switched. These variations have the effect of spreading the frequency of excitation over a wider range, thus reducing the peak harmonic excitation at the switching frequency.
In the preferred embodiment, a table of small, semi-random phase shift offsets is maintained, and an offset from the table is added to the commutation point (i.e., the point or angle at which a phase change occurs in the drive current). In one variation of this embodiment, the offset entries from the table are retrieved and added to the commutation points in a round robin sequence. The number of entries in the table N is preferably greater than the number commutation points P in a single revolution of the rotor, and is a prime number or a number having no common factors with P. The semi-random pattern of phase shifts therefore repeats itself every N disk revolutions. If necessary, N can be made sufficiently large so that the fundamental frequency of pattern repetition from the table is too low to be perceptible.
Preferably, semi-random commutation offsets are not employed during a spin-up period when the electronic drive circuitry is attempting to bring the motor up to its rated operating speed, and are only used after the motor has already reached operating speed. This avoids unnecessary interference with the process of spinning up the disk assembly.
It is therefore expected that an electric motor constructed in accordance with the present invention therefore will emit less noise as perceived by a user, and may have additional collateral benefits due to a reduction in mechanical vibration at the motor switching harmonic frequencies.
The details of the present invention, both as to its structure and operation, can best be understood in reference to the accompanying drawings, in which like reference numerals refer to like parts, and in which: